Oleo strut for airplane landing gears



@et l2, 948 M. R. MULLEN OLEO STRUT FOR AIRPLNE LANDING GEARS 2Sheets-Sheet 1 Filed Dec. 14, 1942 @et l2? i948. M. R. MULLEN 2,45L17OLEO STRUT FOR IRPLANE LANDING GEARS Filed Dec. 14, 1942 2 Sheets-Sheet2 Patented Oct. 12, 1948 OLEO STRUT FOR AIRPLANE LANDING GEARS f lMilton R. Mullen, Philadelphia, Pa., assigner of one-fourth to Wm.Steell Jackson and Joseplf Gray Jackson Application December 14, 1942,Serial No.1468,94n2

- 1 My invention relates to the struts of landing gears used byairplanes.

One purpose of my invention ls to aiord a softer entry into the easementcurve and to smooth out the ripples in the' take ofi and in the easementcurve in landing, which ripples are causedr by unevenness in groundsurface.

A main purpose is to provide a strut cushion offering the generalcharacter of protection which the Wheel tire compression oiers butcapable of very much more extended benefit than can be secured by thetire compression.

A further purpose is to present a. strut cushion comparable in itsaction with that of a tire (except that it is snubbed on the rebound)but capable of larger proportionate compression and hence of graduatedpressures over a much wider pressure range than can be secured in thetire. It might be compared with a spring snubbed on the rebound and inseries with the tire. If the existing throttled liquid action becomparedy with spring action the comparison would have to be qualiedbythe criticism that its action is snubbed in both directions. Thiscauses their action, particularly during taxiingnto be compared with theunsprung action of a flat tire.

A further purpose is to provide an oleo leg with a standard restrictedoil ow through a throttling passage whether uniform orprogressively'changed and to cushion the initial pressure byhydropneumat-ic means operating von the pressure side of a cushioningbulkhead. f

3 claims. (c1. 267-64) tical. effective and reliable and which at thesame f tudinai sections of various forms.

. lparts.

In the drawings similar numerals indicate like Inalmost ail landinggears the first shock of impact with the ground is taken by large tiresat the lower ends of hollow loleo struts. The tires flatten to varyingdegrees but do not change much in resisting internal pressure becausethe volumes of the flattened portions are small as compared with thetotal volumes within the tires.

In series, as it were, with the tires-because it cannot start untilafterthe tires receive the shock A further purpose is to use the oil ofan` oleo leg s for a double purpose, by throttled discharge against anair cushion, and coincidently by unrestricted flow as the hydraulicelement of an hydraulic accumulator.

A further purpose is to soften thel initial shock which takes place whensections of a, landing strut telescope against an air cushionandsubsequently use it to absorb the local shocks from travel over roughground.

A further purpose is to interpose an air cushion .comparable to a tire,but in range and variation infpressure superior to a tire, in seriesbetween the existing landing gear tire and the throttled oil dischargeand air compression of existing collapsible-leg oleo struts.

Further purposes will appear in tions and in the claims.

My invention relates to a method or process of cushioning and also toamechanism by which the method or process may be carried out.

the speclace;

and begin to transmit this shock-occurs a telescoping of hollow oleostrut members by which liquid (preferably oil) in vone of these membersis driven through a restricted opening in what is eilective as abulkhead within 'another`chamber, such as the bulkhead in the othermember.

The opening may be further restricted on rebound in any of the waysshown in later figures. Theliquid progressively compresses, 'andincreases the pressure of, air trapped in the vother member (chamber),rpermitting continued telescoping of' the hollow members andcorresponding movement of the body ofthe plane toward the wheel. vThemovement of the wheel with respect to thel body takes place as fast asliquid from'the high pressure (under) side of the bulkheadpasses'through the orifice orl orifices in the bulkhead against the fair resistance. i l

At the same time that liquid is being forced through a throttlingopening to compress `air in a space, shown at the top of thestrut inFigures 1 and 2,`the same liquid underipressure is used as the hydraulicpart of an hydraulic accumulator whose air compartment, within anlv'enveloping cover, lies within or in communication with the oil on thehigh pressure side of the opening. The

liquid has free access to the air compartment I have preferred .to'illustrate the' invention by partment of the accumulator, up to hundredsof a few forms only, selecting forms which are pracwhen compression ofair is taking place and may or may not have free release to doin-the-main high pressure body of the liquid when the'air is expanding.

Initial pressure may be provided in the air com 3 pounds, so as not tohave further compression of the air in the air compartment take placeuntil this initial pressure has been exceeded. This is a matter ofdesign.l

The oleo leg shown is based upon a tire-supf ported lower telescopicmember and an upper telescopic member with liquid carried by thelprogressi-ve compression of the air in this upper air space builds up anopposing air pressure,

, usually starting from pressure equivalent to the i tire pressure, inthe ratio o! their respective cross sectional areas.

The piston in Figure Z at any suitable point supports the collapsibleair bulb Ztl of an ,air cell unit 2| which cooperates withthe liquidonthe high pressure side of the bulkheadand is complunger within anupper cylinder, I shall so consider them, with the thought, however,that a reversal in so common a feature will not alter the cooperativeeiect, nor will it alter the beneilt of my invention in connection withany such construction.

' Within the body of liquid on the high pressure side of the throttlingpoint, that is, the body of liquid immediately engaged by the plunger todevelop high liquid pressure, or in communication with this liquid, Isupply what I will call a collapsible air cell which may be in the formof a bulb or other segregated container or space which acts as the aircell of an hydraulic accumulator. The high pressure liquid is the liquidof this accumulator and has access freely tothe movable (collapsible)wall of the cell to compress it quickly, but there may be either quickor retarded release of this liquid to permit quick, or

n to provide slower, expansion oi the movable wall 'of such a unit.

The term hydraulic accumulator has another meaning in the art and isused sometimes when the air cell only of the combination is intended,

either use of the term being permissible provided the context indicatethe sense in which the term is used.

' The piston or plungerlt of one oi' what are usually duplicate oleolegs carries a landing wheel ii upon an axle I2. Suitable bearings notshown are provided. Upon the'wheel is mounted a. large pneumatic tire i8whose compression offers the ilrstl yielding impact against the when anairplane lands.

lu order that the lpiston may not rotate .with respect to the cylinderand may be prevented from expulsion from the cylinder, any suitable typeof torsion links is used, of which fragmentary portions are shown in theillustration at I6. These are well known in this art.

InFlgure 2 the lower section, piston I0, telescopes within cylinder l5which normally carries the bulk of the liquid I6.

At or near the normal upper liquid level when the plane is in the air.is located a bulkhead I1 which is perforated at one or more points Il toallowy throttled passage of liquid through the bulkhead into the airspace i9 above the bulkhead. Within this space the liquid acts -as aliquid pis-ton ,to compress air as and when the liquid passes upwardlythrough the bulkhead. The throttling thus produced retards the rateI atwhich the liquid yields to pressure upon it as the impact or weight ofthe airplane during or after landing reacts through'the tire.correspondingly it retards the rate at which the parts telescope. Theliquid ow is further reduced by the i'act that pressed by it. The aircell and liquid under pressure having access to the air cell togetherform an hydraulic accumulator which may or may not be under initial airpressure, as preferred.

For convenience in supporter the air cell unit, I show in Figure 2 oneform or construction by which the piston may be used for that purposewithout interfering with an votherwisefnormal type of piston. It isrecognized as one form only of many permissible supports and is given inillustration and not in limitation. L

In order to avoid need of the large volume oi liquid qwhich would benecessary ii the (lower) plunger were illled with liquid as in Figure 6,a false bottom, as it were, is built into the piston or plunger member,in Figure 2. It carries the air cell unit in a bracket which at the sametime forms a liquid tight bottom for the liquid space.

The Upper end of the piston is hollow and is interiorly hanged at Z2 andthreaded to receive a barrel 123 which is threaded exteriorly at 24 toengage' the interior threads. It is exteriorly flanged at 2c at the topand is faced at 26 to seal against the :laced end 2j of the outer (end)lpiston wall.

The barrel is interiorly langed at 28 at the top and threaded at 29 toreceive a head 30 which may take the form of a threaded disc 3i.

The disc 3| is apertured at 92 to allow liquid to pass downwardly freelythrough the disc.

40 When it is desired to throttle upward liquid :dow

through the disc some of these apertures are closed against returnliquid ow by ap valves 33 which do not interfere with downward 'low.

Whether ilap valves be used or not there is free flow in one direction,notonly through the apertures without aps but through these with flapshaving restricted ilow in the reverseA (rebound) direction.

The free ilow of liquid through the apertures' 32-one way. and withrestricted dow in the reverse direction, illustrated in Figure 2 ,asaccomplished by ilapyalves-produces a snubbing effect on the4rebound'oering a benefit somewhat comparable with that of snubbedsprings in automobile use. lThe compression and expansionof air in theupper part oi' the cylinder'is incapable of this diierential action,relatively snubbing in one direction, because it does not have freeliquid movement in either direction, the liquid being throttled not onlyduringliquid ilow which-compresses the air but inliquid ow as a resultof expansion of the air.

The lower part of the barrel 23 is intriorly flanged at 34 and theilange is upwardly faced to receive and hydraulically seal against theunder face of ange 35 of a bowl 3e, iorming the lower limiting containerwall of the air ycell unit 2 I.

The bowl iii is rigid but is capped by a flexible bulb-like cover 31,anged at 38. Above the cap bulb is a grid 38 which allows acce'ss ofliquid to the bulb freely through apertures ltoo small for parts of thebulb to be forced out through them. The grid is flanged at 40 and boltsIl pass through the lower barrel ange, bowl ilange, bulb flange and gridflange. with any ring washers desired,

not only to hold these Aparts together but to seal against leakage ofliqui-d through the barrel or Inl Figures 5 and d no pin is used and theaperture is uninterrupted in cross section.

into the interior air space of the air unit formed. 4

' The grid protection permits preliminary air pres,-

and the balance of possible gain from the stretch-i ing of the diaphragmas against shortening of the life. These must be appraised anddetermined with regard to the intended use and are, therefore, Viewed byme as items of design. The structure is entirely operative with aflexible cup. Resilience and elasticity are pointed out Vmerely asoffering advantages to be considered as for possible additionalbenefits, against known objections.

The grid is so shaped that initial air pressure within the -bulb willpush the bulb out against the grid where it receives full support fromthe grid but without straining the bulb`and, unless it be the intentionto use a resiliently stretchable bulb such as a rubber or rubber-likepro-duct,l

without stretching the buib material.

It is also the intention that the bowl and bulb shall bear such size andshape relation that within the limits of maximum pressure contemplated,the cup forming the bulb will noi; be forced down into the bowl farenough injuriously to distend the material of the bulb, planning to flexthis material rather than to stretch it, unless the pressures used andother conditions of use permit and the design provide a rubber orrubber-like or otherwise stretchable material. r

The proportions of the piston, cylinder liquid content and the two airspace Volumes are such that the piston cannot at any time strike thebulkhead because of the high air pressure which is developed, and thebulk of air in the two air reservoirs. y

The liquid content lls the barrel about the air cell, the cylinder abovethe piston and varying parts of the air-containing cushion space abovethe bulkhead. The pressure of the liquid presses both ways at the sametime (actually in all directions, of course), tending to compress theair in each of the two air cushions provided.

Each of the air cushions, when compressed oifers resilient reaction tomaintain pressure of liquid upon the other. It is to be noted, however,that as to each of these cushions if given an initial pressure, it doesnot yield further to pressure until a pressure of liquid is reachedwhich exceeds the initial pressurein that cushion.

It will be noted that in Figure 2 a metering pin i6 carried by thepiston progressively reduces the size of the opening i8 as the pistonand cylinder telescope. This movement forces liquid through theperforation. This pin makes the effective section of an orifice anannulus of variable area.

.In Figure 4 the pin 46 is of uniform thickness,`

used for the purpose chieiiy of making the orice an annulus, but in thiscase of uniform crosssectional area.

Access of the liquid to the bulb does not require that the air cell orbulb be actually Within the leg, much less that it be` within the pistonor.

plunger. For example the air cell 2| (Figure 4) may be within a shell yMwholly outside of the telescopic parts of `the leg but hydraulicallyconnected with the cylinder by` a conduit I5 from a point below thebulkhead, as seen in Figure 4. The unit in Figure 4 may contain the sametypes of bowl, bulb, grid, etc., as are used in Figure 2.

Obviously either the upper or lower` of the telescopic parts may supportor contain the cell of Figure 4. Neither need actually house it.

It will be evident that the shell Il may serve the purpose of, andoccupy the place of the grid 39 in Figure 4 provided it be in positionso that the cup or cover 31' will engage the shellas it would engagetl.e grid-with full movement toward the right in Figure 4 and with/whathas been determined to be permissible or desirable stretch if this beintended. .The conduit in Figure 4 is intended freely to communicatewith the liquid space from which it draws when there is no flap valve.However, the benefit of a flap valve 33' may be secured to take care ofthe rebound by having this flap valve cover a portion only of theopening to and from the conduit.

In Figure 4 the shell may occupy the position of the grid, to supportthe bulb with the inlet Y of the conduit the only part having grid barsDiii'erential throttling of the liquid passage through the perforationi8 or |82 may bearranged by a ap valve 332 which provides, predeterminedthrottling or limitation of flow through the bulkhead from the upper airchamber but permits much quicker flow of liquid in an upward directionthan when the air in the upper air chamber is expanding.

In Figures 5 and 6 the supporting structure for the air bulb need not bea closed cylinder but is a mere bracket or group of brackets 41, securedto the bulkhead in Figure 5'and, upside down as compared with this, tothe bottom in Figure 6.

While the seal between the bowl flange and the cover or diaphragm flangemust be hydraulically tight, so that there will be no leakage, the otherconnections have mechanical requirements of strength but do not requireto be liquid-tight. The orifices may be partially closed by ap valvesclosing part of the valve (or with several orifices,

all of one or more orices) to take care of rebound, if such care bedesired, in each of Figures 4, 5, 6, 7 and 8.

In Figures 2 and 4, where a flap valve is used the ap valve 33 maypartially overlap the position of the pin (i. e., extend from one sidenot only to or nearly to the vlargest diameter of pin which will enterthe orifice, but extend along opposite sides of the pin) The opportunityto select bulbs which are flexible merely or are also stretchable andrecover quickly is present in all of the air units shown which are orform parts of hydraulic accumulators and are used ior support of thebulbs either by the grid or shell against excessive stress or by thelimitation against excessive movementtoward the bowl which exists in allof them.

In Figure 6 as a matter of illustration only lof what is permissiblefrom the standpoint that "it will work, but not desirable because ofexcessive amount of liquid which it necessitates, the unit correspondingto the air unit in Figure 5 is placed in the bottom of a hollow piston.

The pacing' of this airv unit in the bottom re quires that the pistonvas well as part of the cylinder be filled with liquid, c )bvjectionablyincreasing weightasV well asrequiring additional liquid.`

Not only is the bulkhead p erforated and lts ,oriilce |83 permissiblyilap-valved, but the upper .end of the piston in tliisilgure isapertured at 32'.` One ormore of the openings are wholly or l coming torest on the tires.

vpartially closed from below by flap valve 33 1n No pin is used and anydesired amount of constriction of the upward ow through the perforation|83 may be provided so as to use the air cell differentially, giving alower rate of constriction during compression of the air than duringexpansion of the air of this cell.v A

Y tected against excessive expansion.

Both air chambers may thus be protected with troduced into the air cellthrough a check valve *Q ne, passing liquid through Aa throttlingopening against air under increasing pressure, ls. designed primarily todissipate energy but nevertheless acts as a cushion 5v and slowly yieldsto pressure, resulting in teleliquid to its air cell on the highpressure side of the throttling point, gives a soft and more quicklyresponsivelcushion which acts to take up the utte due to roughnesses inthe surface of the field, either upon landing or,on taxiing over theeld. It reacts to the same high pressure of liquid within the leg asthat of the liquid which passes through the orifice but in'its actionmuch more nearly resembles the landing tire, than it does theiiuid-throttling, energy-dissipating action of the other form. The aircell preferably has au initial pressure of, let us say, two or threehur-` dred pounds. If under initial pressure it is prol- .In effect,considered just as cushions, the liquid- 42. Similarly in Figures 5, 6,7 and 8 check 25 valves 42 and pipes 43 may be used for a similarpurpose. f i

In Figure 7 an upper hydraulic accumulator air cell v2| is shown with aguard or grid 39 'air cushions with a slug of liquid between, which fair cushion, and is free from damping in its comadapted" to restrainexcessive movement due to internal pressure of the cover or diaphragm312.

The liquid compresses air within an accumulator cell 2|. Initialpressure may be increased by forcing air in through check valve @l2 andpipev 43'. There is no separate bowl. occupies the space at 48 andenters and leaves the apertures 324.

The pistonat its upper endV at 49 terminates in a head which isapertured at 824, 325 of which rebound; 'and some of the grid openingmay be restricted byaps 33' in opening movement.

A floating piston 50 lies within the upper por- The liquid 35 someapertures are unrestricted in both directions 40 vand others in onedirection to eliminate excessive tion of the hollow telescopic pistonand is moved 45 by the liquid under pressure against increasing airresistance. It is returned by expanding aid pression of the lower aircushion although both are preferably damped in rebound. Each acts as anabutment, against which the liquid presses to ,exert the liquidspressure against the other.

'I'he tire acts very well, momentarily, until its small range of totalcompression and increase in resistance is taken up. The quick reactionof the accumulator divides the contraction and expansion with the, tireand, consequently, when there is a partial relief from pressure, theaccumulator tends to make the tire follow the ground more closely thanwould otherwise be the case. For this reason the initial compression ofthe air cell is preferably less than the compression occurring in thetire with the airplane standing, considering the proportion/ ofthe\relative cross-sectional areas.

@The hydraulic accumulator acts spontaneously l and almostinstantaneously in series with the tire, Y

getting its-thrust free from restriction due to constricted orifices.

compresses air within the space 5| Aincreasing 50 pressure from vanydesired initial pressure as determined by insertion of air to the space5| through a check valve. 5H. The usual landing tire is intended to beused.

It will be noted that the hydraulic accumulator acts much like a tirebut with a control of re,

bound which a tire does not have. 'Ifhe accumug energy-dissipatingaction of the throttled liquid In Figure 8 the cylinder and piston intelescopic 55 movement compressor allow to expand-air in one chamberonly and that in an elongated air cell 52 restrained against excessiveexpansion under pressure by a grid 53 and filled with air through lacheck valve connection 54. The grid 4 carries the air unit and a landingwheel is intended to be used.

In operation, when the tire strikes, telescoping of the cylinder overthe piston begins.. Immediately, two cushioning actions start, eachassisting in smoothing out the curve of impact, relieving flow except asit allows a cushioning action preliminary to such cushioning a-sinevitably takes place during liquid flow. It tends to equalize' thepressure upon the liquid and makes more uniform the extent of fiowthrough the orifices.

It will be evidentl that the diaphragm of most of the forms, separatingthe liquidunder pressure from the air of the hydraulic accumulator aircell is just as much a movable air cell wall as is the piston 5|! in theform of Figure 7 and that in each case the air cell wall is accessiblefreely to'the 5 liquid on the pressure side of the bulkhead.

the jar; and the two together result in the plane In Figure 9 is shownanother form of metering pin leg intended to rest upon a pneumatic tirewheel and to connect with/the body of an airplane.V

In this form the plunger 55 telescopes into an outer shell 56' lwhich inits upper end is` intended to contain liquid I5 and an air cell 51 of anhydraulic accumulator. A 'guard 58 for the collapsible part of the cellis fastened in the upper part of the shell and supports a downwardlydirected metering pin 59. The guard is apertured 4atusinri to allow freeaccess of liquid while preventing extrusion of the' cell material whenthe cell i's under initial pressure.

With closing telescopic movement of the parts the liquid I6 is underpressure and brings pressure against the surface of the air cell.

The liquid under pressure is squeezed downwardly now instead of upwardlyas compared with Figure 2, and passes through a throttling opening I8closed to different extents according to the position of the taperedpin. v

As thus described the operation isvvery similar to the operation inFigure 2. However, the liquid which passes through the throttlingopening i8 passes downwardly through a tube 60 rigid with and sealingagainst the under face of the piston 6I. The liquid passes into thelower part of the plunger at 62 where it spreads outwardly and upwardly,following the arrows 63 and brings pressure upon a piston 64.

The piston is annular. It seals internally against the exterior of thetube and externally against the interior. of the plunger, with theelevct that during telescopic closing of the parts and consequent upwardmovement ofthe 4piston in Figure 9, air is compressed within an annularcushion chamber 65 surrounding the tube 60. Ihis chamber is closed atthe top by the plunger head 66, at the outsides by the interior surfaceof the plunger and at the bottom by the upper surface of the piston.

Downward movement of the piston can be stopped by a collar 61,preferably but not necessarily high enough for the piston always to sealat the inside against the tube.

Pressure can be applied initially to the cell through pipe I3' guardedby a check valve I2.

Air under pressure can be forced into chamber flap valve 69 and the onlyform of 'deliberate release available is by withdrawal' oi' the plungerlfrom the shell, and unscrewing the upper part o1' the plunger from itsbase at 10.

'I'he tubing feeding the air cushion 65 may or may not be armouredtubing according to the pressures which are contemplated and the sizeand character of the tubing, but if athin flexible tubing it should atleast be spiralled with wire to maintain it against binding and keep itscoils spaced from each other.

The upper part of the space for liquid surrounding the metering pin mayalso be used as an air cushion, supplying the air to it through pipe 43checked by valve l2'.

In operation the form of Figure 9 is very much like the form in Figure2. With collapsible parts the liquid immediately under pressure has freeaccess to the diaphragm of the upper cushion (the accumulatorl air cell)and restricted access, through the progressively more restrictedopening, down through the tube tothe piston inthe plunger.

Air may accumulate below the piston but if it appear in any quantityit'may be pushed down by the piston and allowed to rise through thetube. This air will act in much the same way as air compressed in thespace 66 surrounding the 10 tube but above the piston, so that it makespractically no difference whether this air be there or not.

Having thus described my invention what I claim as new and desire tosecure by Letters Patent is: y

1. In an oleo strut, a pair of telescopic members arranged one above theother, the lower member having an apertured upper end and a compartmentbeneath the apertures in the upper end, an apertured bulkhead in theupper member,

liquid in the lower member adapted to be forced.

through the apertured bulkhead by closing telescoping movement of themembers and trapping air above the bulkhead, one-way valve mechanismproviding free passage of liquid into said compartment and less freepassage from it and an hydraulic accumulator air cell located in saidcompartment.

2. In an oleo strut, upper and lower telescopic strut members, liquid inthe upper strut member, an air cushion in the upper part of the upper'member compressed by the liquid, an air container within the lowermember, comprising a second cushion, lling means for placing the airunder pressure, whereby with closing movement of the telescopic membersboth air cushions are compressed on the compression stroke from apredetermined initial pressure, one freely and the other subject to thelimitations of a restricting orice, and with opening movement of thetelescopic members the pressure upon each of the cushions is reduced.

3. In an oleo strut, upper and lower telescoping members of which thelower member is the plunger, liquid in the strut, an hydraulic air cellin the upper member above the highest position to which the plunger isintended to reach, and to which-liquid in the strut has free access whenthe strut is closing, an opening within the plunger, a metering pinconverging downwardly passing through'the opening, a tube surroundingthe pin and passing down to the lower part of the plunger and a pistonmovable within the plunger and about the tube, urged upwardly by liquidpassing through the tube and walls forming space above the piston.

IVIILTON R. MULLEN.

REFERENCES CITED The following references are of record in the ille ofthis patent:

UNITED STATES PATENTS Number Name Date.

1,291,016 Kellogg Jan. 14. 1919 1,328,496 Cox Jan. 20, 1920 1,855,064Messier Apr. 19, 1932 1,956,669 Charles May 1, 1934 1,967,641 WallaceJuly 24, 1934 2,106,198 Williams Jan. 25, 1938 2,165,465 l Ehrhardt etal July 11, 1939 2,196,089 Wallace Apr. 2, 1940- 2,216,374 Martin Oct.1, 1940 2,263,710 Wallace Nov. 25, 1941 FOREIGN PATENTS y Number CountryDate 523,283 Great Britain July 10, 1940 845,721

France May 22, 1939 a closed air

